While continuous wave (CW) radars have a number of important advantages over other radar systems for some applications, CW systems have several problems. First, some type of modulation must be applied to the signal to provide target ranging information. Additionally, the radar is transmitting and receiving at the same time, which imposes stringent isolation requirements and, for ground-based radars, generates severe near-in clutter problems. Further, incorporation of the ranging modulation may tend to aggravate the isolation and clutter problems.
Several modulation techniques have been proposed for providing the signal bandwidth required for target ranging. One method incorporates the bi-phase modulation of a CW carrier according to a particular code, such as an m-sequence pseudo-random code. The most commonly used approach, however, is that of the Linear Frequency Modulated (LFM) carrier. This well-known method is discussed in numerous radar texts.
The LFM waveform does have some significant problems. These problems arise from the inherent coupling of the target range and velocity in the radar return signal. This coupling is manifested by a skewed ridge on the LFM waveform ambiguity diagram. In certain applications the effects of the range-doppler coupling can be reduced. In reducing the coupling, the approach usually involves the sequential transmission of different modulated signals. This, in turn, requires two separate sequential detections. Moreover, the problem is dramatically complicated by the presence of clutter, which cannot be neglected for ground-based radar applications. The coupling of the range and doppler terms in the return signal causes stationary ground clutter to appear in the doppler bandwidth of interest. Since these clutter signals may easily mask target returns, special efforts must be taken to eliminate the clutter, such as totally rejecting all returns from one-half of the doppler spectrum.
Ranging techniques are discussed in the literature. For example, "Introduction to Radar Systems" by M. I. Skolnik, McGraw-Hill Book Company, 1962, discusses continuous wave and frequency modulated radar in Chapter 3. In particular, section 3.3, pages 86-103, discloses block diagrams of several CW radar techniques. On page 88 a CW ranging technique is discussed which uses both up and down chirp signals. In this approach, the signals are transmitted sequentially. Furthermore, the sequential technique does not coherently combine the signals, and this results in a loss in detection capability.